Several types of AF function of an imaging device are known. One of them is contrast autofocus. The contrast autofocus is to move a focus lens of a main optical system having an image sensor, calculate a contrast value of a subject image from the image sensor in each lens moving step, and determine the position of the focus lens with the maximal contrast value as an in-focus position.
There has been a demand for increasing an autofocus speed. A known technique is to heighten the speed at which a subject image is read for calculation of a contrast value and thereby increase contrast AF speed.
To heighten the image read speed, for example, not image data of the entire image area but that of a part of the image area needed for a contrast value calculation is read from the image sensor (hereinafter, partial read process). Assumed that the read speed for the entire image area is 30 fps (30 images per second), the read speed for the partial image area can be 120 fps, for example.
The imaging device includes a main display unit on the back of a body to display a subject image acquired via an optical system. Users can take photos while viewing subject images on the display unit. Further, users can know the optimal shutter timing by visually checking an indication for the completion of AF process superimposed on the subject image. Thus, it is preferable to constantly display a subject image on the display unit during shooting operation.
However, there is a problem in the partial read process that a partially read image is not suitable for the user's visual check and image display may be disordered. In view of this, Japanese Patent No. 4050385 discloses an imaging device to temporarily stop the image display on the display unit during the contrast AF process and resume it after the completion of the process, for example.
Such an imaging device, however, still faces a problem that a subject image may move to outside the display area when the image display is resumed after completion of the AF process since the image display is stopped during the contrast autofocus process in which no image data is read from the entire image area.